"She has her child."
Translation:Hon har sitt barn.
It is a difference of gender. It is "sitt" here because "barn" becomes "barnet" in definite form. If the word were to be "katt" instead of "barn," it would be "sin" because "katt" becomes "katten" in definite form. The distinction is identical to using en or ett before the word.
- sin is for singular en-words
- sitt is for singular ett-words
- sina is for plurals
barn is an ett-word, so since barn is the singular form, it's sitt barn.
And barnen is its plural form, so in the plural, it's sina barn.
But it can never be sin barn since words cannot change gender.